1. Technical Field
This invention relates generally to medical devices useful for the repair of nerve defects and, particularly, to a bioabsorbable surgical device useful for spanning a significant nerve gap where the nerve ends may not be easily pulled and sutured together. The term "bioabsorbable" is used herein to be synonymous with the terms "biodegradable" and "bioresorbable", and all of the above terms signify that a material so defined is one which is absorbed in living tissue such that the material will disappear from the site of implantation and be metabolized from the body at a reasonably consistent rate and within a reasonable time period.
2. The Prior Art
When a nerve is lacerated or severed it may be repaired by a common surgical procedure known as nerve repair or, technically, neurorrhaphy. With the aid of microsurgical techniques, direct nerve suture can easily be done without the use of additional devices when there is no nerve missing between the severed or lacerated nerve endings. However, when a portion of the nerve is missing, a nerve gap or nerve defect exists. This situation may be overcome by mobilizing the nerve ends, bringing them together and suturing them if the gap or defect is less than 1.5 centimeters. Fairly good results have been obtained by suturing the nerve ends together in this fashion. However, problems do still exist. The process of direct suturing is limited because it is extremely tedious and time consuming. The use of numerous sutures can cause trauma to the nerve which stimulates the formation of intraneural and extraneural connective tissue, or scar tissue. Invasion of the repair site by connective tissue can prevent the regenerating axons in the proximal stump from entering the microscopic tubules contained in the distal stump. This situation often causes formation of painful neuromas at the suture or nerve graft site. Furthermore, it has been shown that for defects or gaps greater then 1.5 centimeters, stretching the nerve ends and directly suturing the ends together creates tension at the suture line which causes greater scar formation and, thus, providing poor results.
The technique used to treat nerve gaps is termed "nerve grafting". Typically, the nerve graft material is taken from another part of a person's body, genrally a nerve that goes to a sensory area of a lower extremity, such as the sural nerve. The sural nerve is taken from the donor site leaving an area of numbness in the lateral aspect of the patient's foot, a long scar up the patient's leg and the future potential for pain at the site at which the sural nerve graft was taken. It would be desirable to be able to provide a nerve graft material that could provide for nerve growth across a significant nerve gap or defect without using nerve graft material taken from the patient's own body.
Animal (non-human) nerve graft substitutes have also been utilized to provide the necessary spanning of the nerve gap or defect. These nerve heterografts have been sutured to the nerve ends in the same fashion as a human graft. However, these types of substitute nerve grafts suffer from many drawbacks. First, the chances for success in achieving nerve regeneration using such grafts has been extremely unpredictable. Second, there is the potential for an autoimmune response by the body to the foreign nerve graft material.
Recognition of this problem has prompted many researchers to explore alternatives to direct suturing and the use of nerve grafts in bridging nerve gaps or defects and a variety of approaches involving the use of many different types of materials have been experimented with over the past years. Methods and devices have been developed which use both suturing and non-suturing methods to provide a direct connection between the nerve ends. All of these alternatives seek to protect the anastomotic nerve site by wrapping, tubulizing, or otherwise encasing it with a natural or foreign substance, either absorbable or nonabsorbable. However, none of the prior art references disclose a successful device and method which allows a nerve to regrow across a significant nerve gap without the use of a nerve graft or direct nerve end to nerve end suture line.
U.S. Pat. Nos. 4,534,349 and 4,669,474 both to T. H. Barrows disclose a medical device and method of use for the sutureless repair of lacerated, severed, or grafted nerves. The device is a longitudinally-openable, porous, rough-surfaced tube of a molded natural or synthetic absorbable polymer. This device was not designed for the treatment of nerve gaps. It was designed to repair a broken nerve without the use of sutures by approximating the two nerve ends together and holding them together within a rough-surfaced tube. If used in a situation involving a nerve gap, an autogenous nerve graft would be used. The tubular device would encase both the graft and the two nerve ends or two separate devices would be required, one at each end of the graft and respective nerve end. Furthermore, the Barrows molded tube comes in two parts which are then hooked together such that the tube would be fairly rigid which would not permit it to be used in situations where the repaired nerve would be required to go around a corner or be subject to bending forces.
Sutureless tubulization techniques are known to be successful only in the case of very small, single fascicle nerves. The saphenous nerve in rats (0.3-0.5 mm diameter) was transected and repaired with a preformed tube or single leaf of collagen membrane as disclosed by J. M. Rosen, E. N. Kaplan, D. L. Jewett, and J. R. Daniels, "Fascicular Sutureless and Suture Repair of the Peripheral Nerves, A Comparison Study in Laboratory Animals", Orthopedic Review 8 (4), 85 (1979). This method of repair avoids sutures but requires a totally tensionless situation to avoid retraction of the nerve stumps. J. M. Rosen in Orthopedic Transactions 6(1), 75(1982) reports that the peroneal nerve in rats (0.5-1.2 mm in diameter) was transected and repaired with a thin-wllled, extruded tube of polyglycolic acid, cut open longitudinally along one wall. This method also requires a totally tensionless situation and is not advisable in the case of larger nerves since the tight fit required to maintain adequate nerve stump approximation would not provide for the release of pressure created by post-surgical swelling.
U.S. Pat. No. 4,662,884 to L. J. Stensaas, et al. discloses a very similar method of nerve repair (no gap) using a nonabsorbable silicone rubber. The use of silicone rubber as a tube conduit for nerve repair is also not without its disadvantages. Since the rubber is non-absorbable in the human body, it will be necessary to perform a second operation to remove the rubber tube after the nerve ends have regrown together. Silicone rubber has the further disadvantage of being impermeable. See, also, R. D. Midgley, et al. "Silicone Rubber Sheathing as an Adjunct to Neural Anastomosis", Surgical Clinic of North America, 48, 1149 (1968), where they report the use of a silicone rubber tube to accomplish nerve repair (no gap) in dogs.
There have been many experiments performed on regrowing nerves across small or insignificant (less than 1.5 centimeters) nerve gaps or defects. Hakan Molander, et al., "Regeneration of Peripheral Nerve Through A Polyglactin Tube", Muscle and Nerve, 5:54-57(1982), reported satisfactory results in bridging small nerve gaps (7 to 9 mm in length) by use of a biodegradable polyglactin suture mesh shaped as a tube around the nerve defect as a framework for proliferating cells. Molander, et al. further reported in "Nerve Repair Using a Polyglactin Tube And Nerve Graft: An Experimental Study in the Rabbit", BIOMATERIALS 4: 276-280 (1983), that a method of bridging a small nerve length) with a polyglactin mesh-tube gave results essentially no different from a conventional nerve graft. However, Molander was using his tube only on small or insignificant nerve gaps (less than or equal to 1 cm).
There is also extensive literature reporting on the use of collagen tubes with or without a laminin gel to treat nerve defects as disclosed by D. G. Kline and G. J. Hayes, "The Use Of A Resorbable Wrapper For Peripheral Nerve Repair, Experimental Studies In Chimpanzees", J. Neurosurgery, 121, 737 (1946), and by R. Madison, et al., "Increased Role of Peripheral Nerve Regeneration Using Bioabsorbable Nerve Guides In a Laminin-containing Gel", Experimental Neurology, 88: 767-772 (1985). However, with the use of collagen tubes or tubes containing laminin to promote neural growth, it is noted that collagen and laminin are highly immunogenic and that techniques have not been perfected to allow their use in humans without an immune response developing. Furthermore, all of these researchers were using their devices on clinically insignificant gaps of 1 centimeter (cm) or less on lower animal forms and not in primates.
Some researchers have found that nerves will not regenerate across a nerve gap of greater than 10 mm (1 cm). B. R. Seckel, et al., "Nerve Regeneration Through Synthetic Biodegradable Nerve Guides: Regulation by the Target Organ", J. Plast. Reconstr. Surg. 74: 173-181 (1984), reported that in a rat model a nerve gap distance of less than 10 mm (1 cm) is crucial to obtain nerve regeneration across a nerve gap or defect.
However, it has been determined through discoveries made by the present inventors that nerves can regenerate across a significant nerve gap greater than 1 cm. S. E. Mackinnon, A. L. Dellon, et al., "Nerve Regeneration Through a Pseudosynovial Sheath in a Primate Model", Plastic And Reconstructive Surgery, 75: 833-839 (1985), report that the nerve endings in a baboon grew back together over a 3 cm nerve gap through a vascularized pseudosynovial sheath. The pseudosynovial sheath had been grown in the baboon's own body for a six-week period before use on the baboon's severed ulnar nerve. For this to work in a human it would still be necessary to prepare the sheath in the human body before undertaking repair of the nerve defect. This would require at least two operations and include all of the pain and costs associated with two surgical operations. Therefore, it would be highly desirable to develop a synthetic bioabsorbable nerve conduit that could be used in humans to span significant nerve gaps or defects of 1.5 centimeter or greater.
U.S. Pat. No. 3,937,223 to R. W. Roth teaches a partially-compressed, heat-embossed, flexible, tissue-absorbable, compacted, surgical hemostatic felt having specific fiber and density measurements which is in the form of a thin conformable mat. Two related patents U.S. Pat. Nos. 4,033,938 and 3,960,152, disclose bioabsorbable polymers of unsymmetrically substituted 1,4-dioxane-2,5-diones which are broadly stated in col. 9, lines 29-31 and in the bridging paragraph of cols. 9 and 10 ('938) and in col. 9, lines 20-23 and lines 51-65 ('152) to be useful as tubes or sheets for surgical repair such as nerve and tendon splicing. A similar disclosure in U.S. Pat. No. 4,074,366 to Capozza Col. 6, lines 13-16 and 43-57, relates to poly(N-acetyl-D-glucosamine), i.e. chitin. However, there is no enabling disclosure in the specifications or in their Examples as to how such tubes are to be prepared, the characteristics required, or their method of use.